This invention relates to arc discharge lamps and, more particularly, to deuterium arc lamps.
Deuterium arc lamps provide an efficient source of ultraviolet (UV) light and have been found particularly useful in absorption spectroscopy equipment, particularly for biomedical applications. The typical range of the UV spectrum required for deuterium lamp analysis is from about 200 to 400 nanometers (nm), although in some instances wavelengths below 200 nm may be desired. Such applications require high UV intensities from uncontaminated sources. UV intensity is primarily a function of envelope material and thickness, with the purity of the envelope affecting the transmission characteristics. Parameters such as fill pressure and lamp geometry directly relate to UV intensity and define desired starting characteristics of the lamp. Typically, a current regulated supply operates the lamps at voltages between 65-95 volts, with the initial voltage for plasma breakdown being in the range of 250-350 volts. Starting is effected by heating a tungsten filament, typically coated with triple carbonate, to a temperature that provides sufficient thermionic emission to effect avalanching and plasma breakdown. Once the lamp ignites, the anode voltage drops from the initial 250-350 volt level to the above-mentioned 65-95 volt range.
Generally, there have been basic categories of deuterium arc lamps available having various life ratings. User specifications typically define a maximum of 50 percent reduction in intensity over the rated life of the lamp. Investigation has demonstrated that the loss of D.sub.2 is one major cause for this intensity reduction, in addition to suspected losses due to emissive material deposited on the envelope in the viewing area.